An optical printer head is known in which a photoconductor, such as an instant film, is exposed to an image to record it with use of a linear liquid crystal shutter. An example of a scanning head that includes this optical printer head was applied for a patent in Japan (Serial No. 2001-313387). FIG. 13 is a sectional view of the optical printer head described in this patent application, and FIG. 14 is a perspective view of an optical printer that is fitted with the optical printer head.
In FIG. 13, an optical printer head 5 comprises a frame body 12, light source 9, liquid crystal shutter 11, light guide plate 14, cushion 19, head top cover 21, lens array 13, etc. The frame body 12 supports individual parts. The light source 9 is composed of a plurality of light emitting diodes (LED's) 9a, 9b and 9c. The liquid crystal shutter 11 is set in the frame body 12. The light guide plate 14 linearly converges light from the light source 9. The cushion 19 presses down the light guide plate 14 from above. The head top cover 21 overspreads the liquid crystal shutter 11. The lens array 13 is formed of a plurality of lens groups that are set in the lower part of the frame body 12.
The liquid crystal shutter 11 and the light source 9 are connected electrically to each other by means of a head-side electrode 31 of a flexible connecting member 7. The flexible connecting member 7 is made to range from a side face 5a of the optical printer head 5 to a lower surface 5e and is turned back at a turning portion 24. Further, the turned flexible connecting member 7 is fixed to the frame body 12 by means of the elastic force of a spring 26. The end portion of the flexible connecting member 7 opposite from the head-side electrode 31 is provided with a joint-side electrode 32 for connection with an external circuit.
In the optical printer shown in FIG. 14., a scanning head 2 and a control board 3 that has a control circuit are stored in an outer case 1. Attached to the lower part of the outer case 1 is a photoconductor cassette 4 that can be drawn out in the direction indicated by arrow A. A photoconductor top portion 8 of the photoconductor cassette 4 is situated under the scanning head 2.
The scanning head 2 is provided with the optical printer head 5 shown in FIG. 13, two rod-shaped guide members 6 that support the optical printer head 5 for reciprocation in the direction of arrow A of FIG. 14, and the flexible connecting member 7 that is drawn out from the side face 5a of the optical printer head 5 and fixed by means of the spring 26. The scanning head 2 is connected electrically to the control board 3 by means of a curved portion 7b of the flexible connecting member 7. Further, the optical printer head 5 has a structure such that it is movable on and along the guide members 6 by means of support portions 5c and 5d that are formed on its lateral parts with respect to its moving direction (direction indicated by arrow A in FIG. 14).
The operation of this prior art example will now be described with reference to FIGS. 13 and 14. When the control board 3 delivers control signals to the light source 9 and the liquid crystal shutter 11 through the flexible connecting member 7, the light source 9 successively emits lights of three colors, red, green, and blue. Based on image data from the control board 3, the liquid crystal shutter 11 selectively turns on and off a linear pixel column (not shown), and an image of one line is formed for exposure on the photoconductor top portion 8 of the photoconductor cassette 4 through the medium of a linear light receiving surface of the lens array 13 that is situated right under the linear pixel column.
Then, the control board 3 drives a scanning motor (not shown) to move the optical printer head 5 for one line in the direction of arrow A in FIG. 14. Thereafter, the control board 3 controls the light source 9 and the liquid crystal shutter 11 to perform exposure operation for the next line, whereupon the photoconductor top portion 8 of the photoconductor cassette 4 is exposed to an image of the next line. By repeating this operation thereafter, the photoconductor top portion 8 of the photoconductor cassette 4 is exposed to an image for one picture.
Since the resolution of the optical printer normally ranges from about 200 to 300 dpi, the linear pixel column of the liquid crystal shutter 11 is very narrow, having a width of 100 μm or thereabout. If the light receiving surface of the lens array 13 that receives lights from the pixel column of the liquid crystal shutter 11 is substantially as wide as the pixel column, the liquid crystal shutter 11 sometimes may be slightly shifted in the left-right direction in FIG. 13, owing to distortion of the external shape of the frame body 12 or errors in the external dimensions of the liquid crystal shutter 11. Thereupon, the lights transmitted through the linear pixel column of the liquid crystal shutter 11 are entirely deviated from the center of the light receiving surface of the lens array 13. Some of these lights are intercepted by the frame body 12, so that the exposure of the photoconductor is reduced considerably and the image 15, quality worsens.
The above problem may possibly be avoided by making the width of the light receiving surface of the lens array 13 greater enough than the pixel column width. If the light receiving surface of the lens array 13, which is formed of a plurality of groups of lenses in the form of a very thin rod each, is widened, the lens groups are considerably increased in number, so that the manufacturing cost is inevitably rendered very high. If the wide lens array is attached to an optical head printer, moreover, the external size of the head and the head weight increase, constituting a substantial hindrance to the realization of a small-sized, lightweight optical printer.
In the optical printer head 5 shown in FIG. 13, furthermore, the lens array 13 is formed of a large number of arrays of lens elements that form erect equimultiple images. It is designed so that the distance from the light receiving surface or a lens end face on the incident light side to the an object surface is equal to the distance from the light emitting surface or a lens end face on the emitted light side to an imaging surface. This is defined as the imaging distance of the lens array. If the object surface or imaging surface (e.g., photoconductor top portion 8 of FIG. 14) is situated in a position accurately corresponding to the imaging distance, a focused, high-resolution image can be formed. If the object surface or imaging surface is situated off the imaging distance, on the other hand, a defocused, low-resolution image is formed.
In the lens array 13 that receives transmitted lights from the liquid crystal shutter 11 and forms an image, in FIGS. 13 and 14, its object surface is a liquid crystal cell substrate of the liquid crystal shutter 11, and its imaging surface is the photoconductor top portion 8. In order to obtain a high-quality image with high resolution, therefore, the distance from the liquid crystal cell substrate surface of the liquid crystal shutter 11 to the light receiving surface of the lens array 13 and the distance from the light emitting surface of the lens array 13 opposite the light receiving surface to the photoconductor top portion 8 must be made accurately equal to the imaging distance of the lens array 13.
However, the liquid crystal shutter 11 of the conventional optical printer head has a structure such that a polarizing plate is put on a liquid crystal cell substrate that is formed of a glass member and coated with an adhesive agent. It is housed directly in a liquid crystal shutter housing recess 12a that is formed in the frame body 12. Therefore, the polarizing plate engages the bottom of the liquid crystal shutter housing recess 12a. Thus, the polarizing plate that is coated with the adhesive agent exists between the liquid crystal cell substrate and the lens array 13. While the polarizing plate normally has a thickness of hundreds of micrometers or thereabout, the thickness finely varies depending on variation in manufacture, and the thickness of the adhesive agent spread on the polarizing plate also finely varies depending on the state of application. Since the polarizing plate is a resin sheet that has elasticity, moreover, the thickness of the adhesive agent varies after the polarizer is put on the liquid crystal cell substrate, due to a difference in pressure that is produced when it is adhesively bonded to the cell board. Owing to these factors combined together, the distance between the liquid crystal cell substrate surface of the liquid crystal shutter 11 and the light receiving surface of the lens array 13 undergoes an error for each optical printer head. In consequence, the imaging distance of the lens array 13 differs from the distance from the liquid crystal cell substrate surface of the liquid crystal shutter 11 to the light receiving surface of the lens array 13. Therefore, the image that is formed on the photoconductor top portion 8 inevitably undergoes exposure as a defocused, low-resolution image. Thus, the image quality is lowered considerably.
Since the optical printer head is simple in construction, furthermore, it can be miniaturized relatively easily and should be in demand as an article for a mobile printer. Thus, its vertical thickness is expected to be minimized.